From a commercial point of view, lead-acid batteries are the most important rechargeable electrochemical systems because of their high reliability and low cost compared to more recent electrochemical systems such as lithium-ion batteries. They are mainly used to start internal combustion engines, although other applications include the storage of energy produced intermittently, such as solar or wind energy.
Lead-acid batteries typically comprise an electrode assembly containing at least one negative electrode (which is the cathode, usually made of PbO2, when charging the battery) and at least one positive electrode (which is the anode, usually made of spongeous lead, when charging the battery), separated by a membrane which may be made of polypropylene or glass fiber, for instance. Batteries also include a bus-bar, an electrolyte solution (generally an aqueous solution of sulphuric acid) and a housing. Each of the electrodes is carried by a current collector which is usually made of lead alloy such as a Pb/Sb or Pb/Ca alloy.
Research has been conducted in the ten last years, so as to increase the electrical conductivity of the positive electrode of lead-acid batteries, and to avoid the formation of large PbSO4 crystals which prevent lead oxidation into PbO2 during charging, with a view to improving the battery charge/discharge properties and thus the active mass utilization of the positive electrode. To this end, attempts have been made to incorporate carbon nanotubes in the paste used for the manufacture of this electrode. Carbon nanotubes consist of rolled graphene sheets built from sp2 hybridized carbon atoms, which are known to be electrically conductive and stable in sulphuric acid electrolyte environments.
Wang et al (Effect of MWCNTs as Additives in Lead Acid Battery, Journal of Materials Science & Engineering, Vol. 25, no 6, 2007) have thus shown that modified multi-walled carbon nanotubes obtained by acid treatment optionally followed by graphitization, when incorporated at 0.16 wt. % in the formulation of the PbO2 positive electrode, improved the conductivity of the electrode and the utilization of the active material of the battery.
Endo et al (Applications of Carbon Nanotubes in the Twenty-First century, Phil. Trans. R. Soc. Lond. A, Vol. 362, pp. 2223-2238, 2004) also report the addition of 1.5 wt. % of carbon nanotubes both in the positive electrode of a lead-acid battery, to improve its conductivity, and in the negative electrode, to improve the cycle characteristics when embedded in a polymer.
However, carbon nanotubes obtained by chemical vapour deposition (CVD), which is the process commonly used to produce carbon nanotubes on a large scale, have an entangled structure resulting from Van der Waals interactions between individual carbon nanotubes, which prevents their homogeneous dispersion in aqueous solvents such as those used in the manufacture of electrode pastes. This poor dispersion may in turn affect the efficacy of the charge transfer between the electrode and the electrolyte and thus the performance and lifespan of the lead-acid battery.
To overcome this deficiency, it has already been proposed to wrap the carbon nanotubes with surfactants and/or polymers which improve their compatibility with the paste medium.
It has also been suggested to prepare an electrode paste by blending lead oxide with oxidized carbon nanotubes in a planetary centrifugal mixer before adding a synthetic fiber, water and sulphuric acid thereto. This process is also too complex to be industrialized because of the large amount of lead oxide that should be ground.
Therefore, there remains the need to provide a simple and economical process for manufacturing electrode pastes containing carbon nanotubes homogeneously dispersed therein and which enable the manufacture of lead-acid batteries having good performances (high current density and possibly low formation of large crystals of PbSO4) and a high lifespan (good mechanical properties and high number of charging/discharging cycles).
The inventors have now shown that this need may be satisfied by providing a process in which the electrode paste is prepared by mixing a lead oxide/nanofillers composite with the remainder of the paste formulation. Especially good results may be obtained in case this composite is obtained by ball grinding.